The five women were from small-town America but chose to live in the midst of one of West Africa’s most brutal civil wars. Each belonged to the Adorers of the Blood of Christ, a St. Louis–based Catholic order; each had volunteered to live in Liberia, not only as missionaries but also as desperately needed relief workers.

In 1993, the St. Louis Post-Dispatch profiled the nuns. Sister Barbara Muttra, the eldest of the group at 69, ministered to refugees during the height of the Vietnam War before moving to Liberia in the early 1970s. Sister Mary Joel Kolmer, 58, was a cancer survivor who returned to Liberia after surgery to remove a tumor. Sister Agnes Mueller, 62, was both a trained nurse and a theologian who taught aspirant nuns at the sisters’ convent. Sister Shirley Kolmer, 61, who served as a high school principal in Monrovia, advocated forcefully — and successfully — for the nuns’ return to Liberia after fighting between Charles Taylor’s rebels and government troops forced the nuns to flee in 1990. And Sister Kathleen McGuire, 54, the only sister who was new to Liberia, once made a pilgrimage to the graves of five American nuns murdered in El Salvador in 1980. It would be a tragedy the five nuns in Liberia would share, slaughtered 20 years ago last week by men believed to be loyal to Taylor.

Their deaths have gone unpunished, but not for lack of evidence. Investigators in Liberia and the U.S. identified some of the individuals they believed responsible, but for reasons both political and legal, it is unlikely that anyone will ever be brought to justice.

The killings remain among the darkest episodes of the war for both Liberians and Americans. In October 1992, Taylor launched the most notorious offensive in his bid to take power, a fast-moving, multipronged attack called Operation Octopus.

On Oct. 20, 1992, Muttra and Mary Joel Kolmer left their home in Gardnersville, Liberia, to drive a Liberian colleague to his nearby village. They never made it to their destination: the women and the Liberian man were shot to death in their vehicle, along with two African peacekeepers the women picked up along the way.

Three days later, according to testimony the sisters’ order provided to Congress, a rebel from Taylor’s faction identified as Mosquito — arrived at their convent with several fighters, announcing that “he was going to kill the white people.” McGuire was shot first, allegedly cut down by Mosquito as she opened the convent’s gate. Another fighter, known only as Black Devil, then executed Shirley Kolmer and Mueller. Their bodies were mutilated and the women’s vehicle looted from the compound.

The U.S. government responded forcefully upon learning of the women’s murders, according to a declassified State Department cable, warning Taylor directly that it would hold him and his commanders “personally responsible for mistreatment of any American citizens.” Immediately afterward, Taylor denied responsibility; it was a position he maintained throughout his rise and fall as a warlord and President in Liberia. “We had protected Americans throughout that period, and it was very — it was a sad situation even for me,” he said at his trial before the Special Court for Sierra Leone in 2010. Taylor was sentenced to 50 years for crimes against humanity earlier this year. But no one has specifically been held accountable for the death of the nuns.

The pursuit of justice in the killings has always been dependent on U.S. policy on Liberia. During the civil war, when U.S. interests focused on stabilizing the nation, the nuns’ murders went without criminal investigation. After Taylor took power and U.S. policy shifted toward pressuring him from office, Congress voiced renewed interest in identifying the perpetrators.

In 2002, the FBI launched an investigation. A team of agents with an extraterritorial investigation squad combed Liberia and neighboring countries for leads, building a body of evidence and, most important, zeroing in on a suspect. Meanwhile, Liberia launched its own national inquiry into civil-war atrocities. At a January 2008 hearing before Liberia’s Truth and Reconciliation Commission, a former Taylor fighter confessed to his involvement in the murder of three of the nuns. That witness, a low-level Taylor fighter named Morris Padmore, also identified Mosquito as Christopher Vambo, a former Taylor commander who had worked with a security company for a legislator in Liberia’s government.

Following that testimony, Sam Saryon, director of the Liberian national police’s criminal-investigations division at that time, approached Vambo for what he described as an “off the record” conversation. Vambo, who lives in Buchanan, the nation’s second largest city, denied any role in murder of the nuns, saying that “he was trying to rescue them.” “He was not credible,” Saryon tells TIME. “But I could not do anything further with that investigation.”

By then, the FBI team had also completed their investigation — though the lead agent in the case would not comment on whom they sought to indict in the crimes. “We put together what I personally thought was a prosecutable case,” former FBI special agent Christopher Locke, lead investigator on the case, tells TIME.

In April 2010, Locke met with Justice Department attorneys in Washington to learn whether the nuns’ case would be brought before an American court. The Justice Department prosecutors, however, had come across an arcane legal roadblock. The case law surrounding the statute of limitations on federal murder charges was ambiguous. The statute of limitations on federal murder charges had been five years until it was eliminated altogether in 1994. It was unclear whether this change applied retroactively, opening the door to a prosecution’s case becoming invalid if a judge decided the change could not be applied to the 1992 murders. Any indictment ran the risk of extraditing a suspect they believed to be a war criminal to the U.S., only to see him let go on a technicality.

It was a risk that the Justice Department was unwilling to take — even if it meant that all of the nuns’ murderers would remain free. The FBI closed the investigation earlier this year, according to Locke, making any future U.S. prosecution highly unlikely.

“While prosecutions are not always possible in cases such as this, the FBI always continues to diligently work and follow all investigative leads toward the service of justice,” Jacqueline Maguire, a spokesperson for FBI’s Washington, D.C., field office, tells TIME. Similarly, the U.S. Attorney’s office in Washington, D.C., which handled the case, declined to comment.

In Liberia, the search for justice ran headlong into a peculiar politics of the postwar society. Despite the death of nearly a quarter-million people during the war, not a single person has been prosecuted domestically for a crime related to the conflict. President Ellen Johnson Sirleaf, a recent Nobel Peace Prize recipient, has overseen near double-digit growth in the tiny nation’s economy and enjoys international acclaim unparalleled for a Liberian leader. But after six years in power, she appears unwilling to press for prosecutions, and so the issue of impunity will not fade away.

Earlier this month, Leymah Gbowee, the Liberian peace activist who shared the Nobel Peace Prize with Sirleaf, resigned from her position leading the National Peace and Reconciliation Initiative, a role appointed to her by the President. “We have a deficit when it comes to having a moral voice in the country,” she told an audience in Paris, according to the Telegraph.

There appears little interest among Liberian law enforcement to pursue the matter. “Nobody is talking about that nun story anymore in Monrovia,” a Liberian national police official said.

Yet for former special agent Locke, now an attorney in private practice, the case remains unfinished business. “It’s an important message to send to the world: if you kill our citizens, it doesn’t matter if it takes us 20 years, we’ll never give up,” says Locke. “There’s still got to be accountability for their actions, and we’ve missed that opportunity.”